The nature of noise
The frequency determines the ‘pitch’ of the sound we hear. Low frequencies are heard as low pitch sounds, while higher frequencies are heard as higher pitch sounds. Sounds with frequencies much above 20,000Hz, called ‘ultrasound’, are normally inaudible, but the exact range varies from person to person, and will vary for an individual over his or her lifetime, depending on factors such as age, health status and noise exposure patterns.
The range of frequencies in environmental noise is divided into octaves, in the same way as the range of sounds in music is divided up, for example on a piano keyboard. Two sounds are said to be an octave apart if the frequency of the higher pitch sound has double the frequency of the lower one.
For acoustic work, the smaller interval of a ‘1/3 octave’ is often used to provide greater information about the frequency content of a sound. The range of frequencies that we can hear, from around 20 to 20,000Hz, covers many of these third-octave bands. Frequencies lower than most people can typically hear—called ‘infrasound’—are also divided up in this way for acoustic measurements.
Humans can hear sounds over a very wide range of intensities, with the highest pressure level, called the ‘pain threshold’ being around a million times the lowest. Both Sound Pressure and Sound Power scales are used to characterise noise source. The scale which describes this huge range of intensities is the ‘decibel’ scale, which is a non-linear (logarithmic) scale with the unit of ‘dB’. Very quiet conditions may have sound pressure levels of 20dB(A) and a quiet bedroom around 30–35dB(A)
The Decibel scale is based on a lowest sound pressure level of 20 micro-Pascals (µPa); which is about the limit of human hearing sensitivity, generally found in younger people who have not been exposed to loud noises, such as loud music. A pressure change of 20 mPa is tiny—around 2 ten-billionths of normal atmospheric pressure—nevertheless some people can hear it.
Our perception of the intensity of sounds is not constant across this range, but decreases or ‘rolls off’ as the frequencies approach the upper and lower limits of our hearing. In contrast, microphones detect noise in a much more ‘linear’ way, according to the pressure of the sound waves they record; that is, the way sounds are recorded depends only on the characteristics of the microphone and measuring system. For this reason, acoustic readings from sound meters are usually adjusted or ‘weighted’ according to internationally accepted graphs of hearing sensitivity.
At low levels A-weighting approximates sensitivity of human ear to noise and is typically used for a wind farm noise assessment. The weighting shows a sharply decreasing sensitivity to sounds of frequencies less than around 250 Hz.
However, for sound sources that have strong low frequency contents (eg music concerts, some industrial processes), C-weighting is used to reflect people’s perception of those sounds, with the unit of dB(C).
Infrasound can be heard through our auditory systems, but only at high levels. The G-weighting curve, with units of dB(G), describes this behaviour, with a generally accepted threshold of about 85dB(G).
As noted above, the directly recorded data depend only on the instrument characteristics; so they are often described as being ‘non-weighted’ or ‘Z-weighted’.
The characteristics of a microphone may become important at very low environmental noise levels, where ‘self-noise’ or the ‘noise floor’ may interfere with measurements. This is simply the level of electronic noise that characterises any measuring instrument. It is noted elsewhere in this report that the microphones used for infrasound measurements showed significant ‘noise floors’ for higher audio frequencies, which made calculation of A-weighted noise levels difficult for some indoor measurements. However, this was not so for the lower frequency and infrasound bands, so calculations of C-weighted and G-weighted levels were unaffected.